In the related art, electrofusion welding is commonly used to join pipes, such as plastics material pipes for water or natural gas, into a pipe fitting. A wire filament (i.e. a resistance heating wire) is securely embedded into the fitting. In use, the pipes are inserted into the fitting, suitably with a tight friction fit, and the wire filament is heated by passing current through it, until the pipes are welded into the fitting with a strong, reliable, gas-impermeable joint.
Traditionally, the wire filament is first formed into a coil or sleeve and is then molded into the fitting. However, more recently, various forms of wirelaying apparatus have been developed that lay the wire directly into the fitting. These wirelaying apparatus produce superior fittings but face a number of difficulties, particularly when it is desired to produce more complex wirelaying configurations.
WO-A-99/33619 (MSA Engineering/Jones, et al.) discloses a wirelaying apparatus wherein the wire filament is laid into the interior bore surface of a cylindrical pipe fitting. A tool head carries a wirelaying tool that has a cutting tip to open a cut in the surface of the fitting. The cutting tip also has an aperture that delivers the wire directly into the cut through the cutting tip. A flap closer then closes the cut over the inserted wire, all in one integrated tool. The fitting is rotated and the tool head advances into the fitting along a linear path to form a helical wirelaying pattern. This unidirectional or “single” wirelaying process lays the wire in a single direction. It is only possible to alter the pitch of the helical path by altering the rate of the relative linear advancement of the tool head with respect to the fitting.
WO-A-99/33619 also discloses another embodiment of the apparatus for “double” or bidirectional wirelaying. Here, in this alternate embodiment of the related art, the cutting tip is a separate component that is rotatable about the wire filament using a rack and pinion arrangement, such that a cutting direction may be changed whilst the cutting tip is still engaged in the surface of the fitting. This embodiment allows wirelaying in a double helical path. However, the cutting tip is vulnerable and may break. If the cutting tip breaks, the wirelaying process must be aborted for that fitting and the process restarted with a new cutting tip and a new fitting, which is time-consuming and wasteful.
Also in the related art, WO-A-97/18512 (Compucraft/Friedman, et al.) discloses an apparatus for embedding a wire into many different types of fittings, including laying wire into a saddle coupler type pipe fitting using a more complex bi-spiral pattern. The apparatus includes a multi-jointed arm that supports interchangeable heads. First, the head carries a laser-powered heat grooving mechanism that applies concentrated heat to the surface to form a groove in the desired pattern. Then, the heat grooving mechanism is swapped for a wire inserting mechanism which retraces the same pattern over the surface while pressing the wire into the groove. However, the apparatus is impractical. Despite these and many other developments in the related art, in practice most saddle couplers are still produced using fusion pads. Here, U.S. Pat. No. 4,684,428 (Ewen et al) discloses one example method for making such fusion pads for a saddle coupler.
Hence, there is still a need to provide an improved wirelaying apparatus and wirelaying method for laying wire into a workpiece which has complex contours (such as a saddle coupler) and/or in a complex pattern (such as a bi-helical or bi-spiral pattern).